1. Field of the Invention
The present invention relates to a water-free, water-washable, energy-curable, polymer-forming composition, especially useful as a print screen coating, and a method for applying same.
2. Background of the Art
In silk screen printing, the ink is forced onto a printing substrate through a stencil, or "mask", having a porous screen area configured in the shape of the indicia to be printed, such as letters or graphic images. The printing substrate can be paper, textile, metal, ceramic, polymer film, and the like. The screen can be a gauze or mesh fabricated from metal, textile fabric such as silk or cotton, or various polymer materials.
The mask is generally prepared by coating a screen with a curable composition, curing the composition, and then engraving the indicia. The engraved areas are porous, thereby permitting ink to be forced through the screen onto the printing substrate to print the indicia.
After printing, the ink on the substrate is cured or hardened by any of several methods such as, for example, exposure of the ink to energy such as heat or radiation (e.g. ultraviolet, electron beam, and the like), evaporation of a solvent in the ink composition, or oxidation hardening of drying oil components (e.g linseed oil, tung oil), and the like.
The three main technologies being practiced today which make up the bulk of the coatings and inks include solvent borne, water borne, and zero volatile organic compounds (VOC). Solvent borne and water borne systems produce coatings which are washable. Water washability is a desired feature of a coating composition since the coating application equipment needs to be cleaned for reuse. However, there has been a technological push to eliminate organic solvents and water in such compositions. Organic solvents present environmental health concerns. And both solvent based and water based systems are energy intensive, requiring drying ovens to remove the solvent or water. For example, thermally induced drying and curing of coated screen fabric typically requires about 7,000 to 12,000 kilojoules of energy per kilogram of fabric as well as a long curing time, typically several hours Consequently, what is desired is a waterless, yet water-dispersible, zero VOC composition which would be particularly useful as a coating for a print screen.